Conventional circular knitting machines include a rotary needle cylinder which may carry a plurality of magnetically actuable jacks or rocking pressers that transmit motion to the needles. The desired pattern to be manufactured is controlled by a coded punched tape or similar program carrier which outpulses knitting and non-knitting commands in the form of impulses to an actuating electromagnet. In the type of machine of particular interest in the instant application, the magnetic field of such electromagnet cooperates in a differential manner with that of a permanent magnet.
In this arrangement, the permanent magnet normally acts on the magnetic portions of successively engageable rocking pressers, which move close to the permanent magnet by the help of a cam. Said magnet attracts and holds such presser against a restoring force thereby situating such presser in a non-knitting position. When a knitting command is outpulsed from the coded tape, the electromagnet is actuated to weaken the attraction force of the permanent magnet so that the rocking presser can move in the direction of the restoring force to a knitting position to operate the associated needle.
In presently known arrangements of this type, such as disclosed in Ribler U.S. Pat. No. 3,605,448, the coil of the electromagnet is generally wound around one soft iron pole piece of the permanent magnet as well as around the core of the electromagnet so that the fields of the permanent magnet and the electromagnet are strongly coupled. It has been found that this scheme is highly inefficient, since the actuation of the electromagnet not only markedly weakens the magnetic field in the pole pieces of the permanent magnet but also serves to change the point of the non-linear B-H curve of the permanent magnet at which such magnet operates. The result of such change is a significant increase in the reluctance of the composite magnetic circuit of the electromagnet, the permanent magnet and the rocking presser, which in turn increases the amount of magneto-motive force necessary to support an actuating flux level in the jack. Accordingly, an unsatisfactory high degree of loading is imposed on the driving circuitry for the actuator.